1. Field of the Invention
The present invention relates to the oxidation of hydrocarbon compounds. In particular, the invention relates to a method for essentially completely oxidizing hydrocarbon compounds to carbon monoxide, carbon dioxide, and water in the presence of a carbonaceous catalyst.
2. Description of Related Art
Hydrocarbon compounds are useful for a number of purposes. In particular, hydrocarbon compounds are useful, inter alia, as fuels, solvents, degreasers, cleaning agents, and polymer precursors. The most important source of hydrocarbon compounds is petroleum crude oil. Refining of crude oil into separate hydrocarbon compound fractions is a well-known processing technique. Also, many uses of hydrocarbon compound yield gaseous and liquid streams containing hydrocarbon compounds. These streams must be discarded or processed for re-use. Thus, there are many opportunities for hydrocarbon compounds to escape into the environment.
Hydrocarbon compounds often are discarded improperly, e.g., without regard for the damage the compound may do in the environment. For example, hydrocarbon compounds often are disposed of merely by discarding the compound into the environment. Typical of such disposal techniques are venting to the atmosphere, terrestrial burial (in containers or not), discharge into the open ocean, burning, and deep-well injection. These techniques contaminate soil, groundwater, and air with hydrocarbon compounds.
Processing and storage of hydrocarbon compounds often leads to contamination of the environment through accident, leaks, and evaporative losses, even though the processor or user of the compounds exercises the utmost care in handling the compounds. For example, accidental spills often contaminate soil, groundwater, and the air. Similarly, leaks from processing equipment allow hydrocarbon compounds to escape into the environment. Hydrocarbon compounds also escape into the environment during use, e.g., by evaporation or spillage, or by design, such as during evaporative drying of protective coatings such as oil-based paints and enamels.
Hydrocarbon compounds not only are deleterious to the environment, but also can be hazardous to human health. For example, many hydrocarbon compounds are mucosal irritants, and some are known or suspected carcinogens.
It is desirable, therefore, to remove hydrocarbon compounds from the environment. In particular, it is desirable to remove such hydrocarbon compounds from air supplied as breathing air to, for example, personnel chambers, portable air packs, and the like. Examples of such personnel chambers include process plant control rooms and other controlled-environment rooms, such as the `clean room` in a silicon chip manufacturing plant.
Methods for eliminating hydrocarbon from the environment are known. For example, catalytic incinerators are known in the art. Various catalysts are used in such incinerators. Many times, the hydrocarbon to be destroyed is removed from a liquid by countercurrent stripping with air, nitrogen, or other gas stream which will carry hydrocarbon. Also, hydrocarbon often is removed from soil by passing a gas, such as air, through the contaminated soil. The gas containing hydrocarbon vapor then is passed over the catalyst, typically at elevated (significantly above 250.degree.-300.degree. C.) temperature, with residence times sufficient to oxidize the hydrocarbon. In addition to the additional energy cost incurred in heating catalyst, reactants, and inert material carried with the reactants (such as nitrogen with the oxygen in air), such high-temperature methods require that the material of construction of the processing apparatus be capable of resisting the temperature utilized. Further, at higher temperatures, there exists the possibility of producing noxious or deleterious compositions, such as NO.sub.x if nitrogen is present during the oxidation, which essentially are not produced at lower temperatures.
A catalyst for complete oxidation of gaseous hydrocarbon mixtures is disclosed in DD 280,395. The catalyst comprises an oxidation component, preferably CuO, and an adsorption component, preferably an aluminosilicate. Processing temperature preferably is 407.degree.-577.degree. C. A plurality of catalysts comprising noble metals also is known. For example, catalyst comprising a platinum-group element on a high purity magnesia single crystal fine powder substrate, or the magnesia alone, is disclosed in JP 03/118,835 (1991). This document discloses that the substrate has a specific surface area of between about 5 and 170 m.sup.2 /g. In JP 03/122,402 (1991), a catalyst having a first layer consisting of powdered magnesia single crystal powder to which a platinum-group metal has been added, is layered with undoped superfine magnesia powder. Apparently, the doped catalyst is active at temperatures between about 300.degree.-800.degree. C.; the undoped catalyst is active at temperatures between about 700.degree.-1500.degree. C. This system is said to be suitable for completely burning lower alkanes at a temperature of at least about 300.degree. C. in the first stage without NO.sub.x emissions.
The catalysts disclosed in JP 02/169,029 and 02/169,030 (1990) comprise noble metal on an inorganic fibrous carrier. The carrier can be alumina, silica, zirconia, titania, and suitable blends. The catalyst is said to provide high activity because the noble metal is uniformly dispersed and because the substrate is resistant to degradation of the porosity, and thus maintains high specific surface area. The noble metal-containing catalyst disclosed in SE 464,392 also is said to retain activity because it is resistant to sintering. The catalyst substrate is aluminium oxide having a specific surface area less than 75 m.sup.2 /g, in which at least 50 percent of the surface area contributed by the pores is contributed by pores having diameter greater than 100 Angstrom. The noble metals are deposited on the substrate.
According to U.S. Pat. No. 4,977,128, high specific surface area is retained, even at temperature exceeding 1000.degree. C., by catalyst comprising platinum or rhodium deposited on a substrate containing aluminium and barium. Steam treatment of a substrate to increase the specific surface area and therefore the activity of the resultant catalyst, is disclosed in JP 02/143,010 (1990). The substrate is thermally conductive; the raw material for the substrate comprises an element selected from the group including manganese, magnesium, iron, copper, cobalt, and chrome. JP 02/078,436 (1990) discloses a three-component, layered catalyst for use in a gas turbine combustor comprises rare earth, alkaline earth metal, and their oxides; magnesium, silica, and their oxides; and heavy metals and their oxides; all on a porous support.
Researchers also have studied structurally-modified catalysts, typically for catalysis of reactions which proceed at high (above about 500.degree. C.) temperature. Noble metal (platinum, palladium) supported on alumina show measurable methane oxidation activity at 300.degree. C., but require a temperature of about 480.degree. C. to achieve complete oxidation, according to Briot, Catalytic oxidation of methane over palladium supported on alumina, Applied Catalysis, 68 (1991) 301-314, and a platinum/nickel/alumina requires a temperature of 430.degree. C. to oxidize a hydrocarbon mixture, according to Agarwal, Deep oxidation of hydrocarbons, Applied Catalysis A: General, 81 (1992) 239-255. Agarwal also discloses that a ceria-promoted hopcalite (mixture of manganese and copper oxides) catalyst has significant oxidation activity at temperature exceeding 300.degree. C. Significant activity loss was experienced at temperatures less than 300.degree. C. The hydrocarbon stream comprised water and a mixture of nine hydrocarbons having a total concentration of 500 ppm.
Research also has been directed to alumina-supported catalysts, in which about 5 wt percent CuO is supported on alumina or on ZnAl.sub.2 O.sub.4. Such catalysts require a reaction temperature of at least about 347.degree. C. to achieve even minimal conversion of methane. Marion, Physicochemical Properties of Copper Oxide Loaded Alumina in Methane Combustion, J. Chem. Soc. Faraday Trans., 86(17) (1990), 3027-3032, and Marion, Catalytic Properties of Copper Oxide supported on Zinc Aluminate in Methane Combustion, J. Chem. Soc. Faraday Trans., 87(11) (1991), 1795-1800. Oxides of cobalt, copper, and chrome also have been deposited on alumina substrate for use as catalyst for methane oxidation.
Adsorbents for hydrocarbons also are available. Methods for regenerating saturated adsorbents used to remove harmful hydrocarbon compounds from water, soil, or air are known. Typical regeneration methods include steam regeneration and solvent regeneration. These methods have the drawback that although the adsorbent may be regenerated, the steam or solvent effluent stream remains contaminated with the harmful hydrocarbon compound and must be treated before disposal.